1. Field
The present invention relates to a cooling structure for a bearing device and, more particularly, to the cooling structure for a main shaft in a machine tool and the cooling structure in the bearing device that is incorporated in the main shaft.
2. Description of the Related Art
In the main shaft device employed in the machine tool, incompatible technologies of suppression of the temperature rise in the main shaft device and of a speed-up of the main shaft device for increasing the machining ability, both required in bearings used to support a main shaft, are desired for in view of the necessity to secure the machining accuracy and also to increase the machining efficiency. Accordingly, in recent years, various new techniques of cooling the bearings and lubricating them come to be introduced.
Specifically, JP Patent Publication No. 2000-161375 discloses the method of cooling by rendering a cold blast to form a spiral flow by injecting such cold blast into a space, which is delimited between, for example, two bearings, at an angle relative to the direction of rotation. JP Patent Publication No. 2000-296439 discloses the use of an air outlet port for air cooling an inner ring spacer, which port is formed in the main shaft or a bearing housing so that an inner ring of the bearing can be indirectly cooled by blowing the inner ring spacer with a compressed air to thereby air cool the latter. If the inner ring is cooled, there is two functions, i.e., a function of dissipating generated heats by means of the cold blast and a function of reducing the bearing preload through the reduction of the inner ring temperature, and a cumulative effect of suppressing the bearing temperature can be expected by means of those functions. JP Patent Publication No. 2012-026496 discloses the use of a ceramic material, which has a density lower than that of a steel material and also has a high modulus of elasticity at a low linear expansion, as material for the inner ring so that the preload can be relieved while the amount of expansion of the inner ring in a radial direction is reduced.
Also, so long as the method of lubricating the bearing is concerned, Japanese Patent No. 4261083, for example, discloses a technique concerning the air oil lubricating method that can be suitably employed for a high speed operation. This known technique is such that an inclined surface area continued to the raceway surface is provided in an outer diametric surface of the bearing inner ring and a ring shaped nozzle member is provided along the inclined surface area with a gap defined therein. The nozzle member is provided with a nozzle for discharging an air oil towards an inner ring inclined surface area and, accordingly, an oil contained in the air oil discharged from the nozzle can be attached on the inclined surface area assuredly. The oil so attached forms an attachment flow by the effect of an attachment force and a centrifugal force brought about by the rotation, and can be assuredly introduced into the bearing. Therefore, along with the speed-up of the bearing, reduction to a noise is made possible as compared with the conventional oiling method.
If the technique of the above discussed air oil lubricating method is applied to a bearing device in which a plurality of rolling bearings are juxtaposed in an axial direction, the result is such as shown in FIGS. 81 and 82 which show as, for example, a suggested example 1. Namely, as shown in FIG. 81, the bearing device includes two axially juxtaposed rolling bearings 101 and 101 and an outer ring spacer 104 and an inner ring spacer 105 that are interposed respectively between outer rings 102 and 102 of each of the rolling bearings 101 and 101 and between inner rings 103 and 103 of each of the rolling bearings 101 and 101. In the example as shown, the rolling bearing 1 is in the form of an angular contact ball bearing. Each of those rolling bearings 101 and 101 is used with the initial preload applied by means of a widthwise dimensional difference between the outer ring spacer 104 and the inner ring spacer 105.
As clearly shown in FIG. 82, the outer ring spacer 104 is comprised of an outer ring spacer man body 110 and a pair of nozzle members 111 and 111, and a nozzle 112 for supplying the air oil towards the associated rolling bearing 101 is provided in each of the nozzle members 111. Respective discharge ports 112a of the nozzles 112 are opposed to a shoulder surface area (inclined surface area) 103b of the inner ring 103 via a gap δ. The shoulder surface area 103b of the inner ring 103 is formed with a plurality of annular recesses 120 that are juxtaposed in a circumferential direction at respective axial positions confronting the discharge ports 112a of the nozzles 112.
The air oil for use in lubricating the bearings is, after having been supplied from an external air oil supply device 145 shown in FIG. 81, discharged from the nozzle 112 by way of a supply port 140, then through a supply hole 147 and finally through an introducing passage 113 within the outer ring spacer 104. The oil contained in the air oil so discharged is blown onto the inner ring 103 at the annular recess 120 of the inner ring 103 and is then attached on the inner ring 103. The oil so attached forms an attachment flow by the effect of the attachment force and the centrifugal force developed as a result of rotation of the inner ring 103, and is guided along the shoulder surface area 103b, shown in FIG. 82, towards the side of a raceway surface 103a and is finally provided for lubrication of the rolling bearing 101.
FIG. 83A illustrates a sectional view showing a cooling structure for the bearing device designed in accordance with a suggested example 2, FIG. 83B illustrates a sectional view showing, on a large scale, a main portion of the cooling structure in FIG. 83A and FIG. 84 is a cross sectional view taken along the line 84-84 in FIG. 83A. In the structure shown in FIGS. 83A, 83B and 84, in which a cooling air is injected onto an outer diametric surface of an inner ring spacer 180 so as to be oriented in a rotational direction of the shaft to thereby cool the bearing through the inner ring spacer 180, test results of the temperature of each of the bearing inner ring 181 and the outer ring 182 and the noise level at that time, which are obtained through experiments conducted in relation to the rotational speed, are shown in FIGS. 85 and 86. In FIGS. 85 and 86, white plots pertain to the results with no cooling effect and black plots pertain to the results with the cooling effected. As shown in FIG. 85, it is suspected that with the air cooling performed, the temperature of each of the inner ring 181 and the outer ring 182 is lowered during the operation and the increase of the bearing preload is suppressed. It is, however, to be noted that the noise level, when the air cooling is performed, is increased by 20 dB (A) at most as compared with that exhibited when no air cooling is performed.
FIG. 87 shows, as a suggested example, 3, the bearing device J in which four rolling bearings 1A, 1B, 1C and 1D, each in the form of an angular contact ball bearing, are arranged in parallel combinations, the left side two rolling bearings 1A and 1B in one combination and the right side two rolling bearings 1C and 1D in the other combination, and the two intermediate rolling bearings 1B and 1C are held in back-to-back combination relative to each other. In such a bearing device J, supply and discharge passages for the air oil AO and the cooling air A will be such as discussed below. Namely, so long as the air oil AO is concerned, as shown by broken line arrows, the air oil AO is supplied from oil supply ports (not shown), which are provided in axially opposed ends of an intermediate outer ring spacer 4M, towards the two intermediate rolling bearings 1B and 1C and, also, from oil supply ports (not shown), which are provided in the left and right side outer ring spacers 4L and 4R, towards the outer rolling bearings 1A and 1D. On the other hand, so long as the cooling air A is concerned, as shown by the solid line arrows, the cooling air A is discharged from a cooling air discharge port 11M, which is provided in the intermediate outer ring spacer 4M, towards between the intermediate outer ring spacer 4M and the inner ring spacer 5M and, also from cooling air discharge ports 11L and 11R, which are provided in the left and right side outer ring spacers 4L and 4R, towards between the left and right side outer ring spacers 4L and 4R and the inner ring spacers 5L and 5R. The air oil AO after the oil for lubrication has been supplied to the rolling bearings 1A, 1B, 1C and 1D, and the cooling air A after the bearing device J and a main shaft 7 have been cooled, are discharged to the outside of a main shaft device from exhaust ports (not shown) provided in axially opposed ends of each of the outer ring spacers 4L, 4M and 4R.
In the case of the bearing device J shown in FIG. 87, air of the air oil AO, which has been injected from the oil supply port in the intermediate outer ring spacer 4M, flows in the same direction as the cooling air A discharged from a cooling air discharge port 11M of the intermediate outer ring spacer 4M. However, the cooling air A, discharged from the cooling air discharge ports 11L and 11R of the left and right side outer ring spacers 4L and 4R, flows in part towards the intermediate rolling bearings 1B and 1C. Thus, the cooling air A discharged from the cooling air discharge ports 11L and 11R of the left and right side outer ring spacer 4L and 4R flows in a direction counter to the direction of flow of the air oil AO which is injected from the oil supply port of the intermediate outer ring spacer 4M. Therefore, the air cooling air A and the air oil AO collide with each other at an axially outer side portion 140 of the intermediate rolling bearings 1B and 1C and, consequently, it occurs that the flow of the air oil AO becomes not stable. Once this occurs, the oil of the air oil AO will no longer be supplied sufficiently towards the axially outer side portion 140, resulting in the occurrence of an excessive temperature rise.
JP Patent Publication No. 2000-161375, JP Patent Publication No. 2000-296439, JP Patent Publication No. 2012-026496, Japanese Patent No. 4261083
To speed up the bearing, the suppression of the bearing temperature during the operation and the assured supply of the lubricant oil are essential requirements. Since the bearing temperature suppressing technique described in JP Patent Publication No. 2000-161375 and JP Patent Publication No. 2000-296439 referred to above is the cooling of the bearing by the use of a compressed air, there is a problem that a substantial amount of air is consumed, while the device can be configured at a relatively inexpensive cost.
On the other hand, the bearing preload during the operation is affected by the difference in amount of radial expansions of the inner and outer rings. In general, if the inner and outer rings, each made of a steel material, are used to operate, the heat generated in the inner ring is difficult to be dissipated as compared the outer ring side in which the bearing box is forcibly cooled by the oil or the like. As a result, the temperature of the inner ring is higher than that of the outer ring. Accordingly, the amount of expansion on the inner ring side tends to become large as compared with that on the outer ring side due to that resulting from the heat generation in cooperation with the centrifugal force. This constitutes a major factor by which the increase of the preload during the operation may result in.
JP Patent Publication No. 2012-026496 referred to above is aimed at relieving the preload by suppressing the inner ring expansion amount during the operation with the use of the inner ring made of a ceramic material and with the utilization of advantages of the ceramics including the low density, the low linear expansion and the high modulus of elasticity. However, even though the ceramic material is used for the inner ring, the inner ring expansion during the operation tends to be largely affected by the expansion brought about by the temperature rise of a shaft and, accordingly, an effect to relieve the preload of the bearing is low while the advantages of the ceramic material are no longer utilized.
Also, although the lubricating method disclosed in Japanese Patent No. 4261083 referred to above may bring about a favorable result so long as the lubrication is concerned, further increase of the cooling effect so as to meet with the further speed-up is demanded.